class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 }


public class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        if (list1 == null) {
            return list2;
        }
        if (list2 == null) {
            return list1;
        }
        if (list1.val >= list2.val) {
            list2.next = mergeTwoLists(list1, list2.next);
            return list2;
        } else {
            list1.next = mergeTwoLists(list1.next, list2);
            return list1;
        }

    }
}
